1. Field of the Invention
The present invention relates to presses for the fabrication of panels of pressed chips, of pressed fibers, or of plywood and the like, and it relates in particular to continuously operating panel presses having cooperating upper and lower endless conveyor bands moving through an adjustable work gap between a press table and a vertically movable upper press spar.
2. Description of the Prior Art
A known panel press of the type mentioned is described in co-inventor Heinrich Pfeiffer's U.S. Pat. No. 3,852,012. It features an open machine frame with horizontally extending upper and lower fixed supports carrying between them an upper, movable press spar and a lower, stationary press spar or press table. The two elongated press spars carry each a conveyor with a flat endless conveyor band running between a drive drum and a reversing drum mounted on the extremities of the press spars. The lower strand of the upper conveyor and the upper strand of the lower conveyor define between them a continuously moving work gap in which the press work--a stack of panels, or layers of raw material and adhesive--is compressed and cured under the application of heat.
In general, the production of chipboard panel, fiber panels, plywood panels, plastic multi-layer panels and the like, but also the application of linings, require the use of high pressure as well as high heat, if the quality standards of high uniformity and resistance are to be consistently met. Some presently known single-level heater pad presses of the intermittently operating type do achieve these quality standards, but their personnel requirements are high and their productivity is obviously limited by the need to proceed by intermittent operating steps.
This shortcoming is overcome with the conveyor-equipped, continuously operating panel press described above, where the press work is continuously advanced by means of the conveyor bands, while heat and pressure are applied to it. Such a press is capable of producing, for instance, a continuous finished chipboard panel of high quality with a minimum of cutting waste. However, the continuously operating panel press has a major limitation, inasmuch as the required pressure creates considerable frictional resistance between the advancing steel bands of the conveyors and the cooperating supporting surfaces on the upper press spar and on the press table. The maximum permissible pressure is thus limited by the tensile strength of the steel bands and/or by the friction drive conditions between the conveyor drive drums and the steel bands. And because the narrowing work gap is essentially fixed for a given run, it is the advancing steel bands that actually have to create the pressure against them. These bands, however, have manufacture and design related limitations to their gauge, so as to in turn limit the maximum applicable pressure against the press work.
Another important operative parameter is the duration of compression, during which time the adhesives in the work are to be cured. This factor, in conjunction with the amount of heat that has to be transferred to the press work, determines the necessary length of the panel press. The length of the work gap thus becomes a crucial feature of this type of press. The longer this gap, the more heat can be transferred to the work, but a longer gap means a lower specific pressure per unit area in the work gap, given the same overall pressure between the press spars. A lower specific pressure, on the other hand, affects the density and the overall quality of the finished product adversely. The optimal length of the work gap must therefore be a compromise between the desire for a high specific pressure, which calls for a short press gap, and a high heat input which is favored by a long gap.
An additional problem encountered with continuously operating presses, especially in connection with the production of chipboard panels, relates to the occurrence of fluctuations and irregularities in the chip layer, when deposited, so that it becomes necessary to reduce the conveyor traction, and with it the work pressure, in order to avoid the risk that the conveyor bands either break or slip on the drive drums.
It has already been suggested to modify the continuously operating panel press for higher work pressure, by using balls or rollers, or even roller chains, which either travel with the endless bands, or are arranged stationarily behind the steel bands. But, even these measures have failed to achieve the high degree of compression and uniformity which is necessary for an economic production of chipboard panels. While a higher compression is indeed achieved this way, the heat transfer is now poorer and the conveyor bands are subjected to higher stress and strain, because of the accentuated point or line pressures on the bands. The result is not only a greatly increased wear on the steel bands, but also a deterioration of the product quality, whose surface is no longer perfectly flat and even. Lastly, this modified continuously operating panel press is complex in structure and costly to build and predictably, subject to considerable downtime due to malfunction.
It has heretofore not been possible, in any of the known continuous panel pressing installations, to independently control the work pressure, by adjusting it as a function of the curing process inside the press work, in the manner in which this is possible with known intermittently operating single-level or multi-level panel presses, where no conveyors are used. Such attempts tend to greatly increase the friction against the steel bands to an unacceptable level.